PURPOSE: To obtain a high quality oblique incident optical measuring apparatus in which a plurality of measurements are not required to be coupled and the image of which is not distorted.

CONSTITUTION: The interferometer comprises a coherent radiation source laser 10, and a beam splitter 16 for splitting the laser output beam into a sample beam A and a reference beam B. A test surface 18 is disposed at a first angle θin the sample A, such that the sample A impinges obliquely on the test surface 18 and reflected therefrom. Reference and sample beams reflected on the surface are coupled through a beam coupler 22, and an image plane 24 is arranged at a second angle θ', with respect to the coupled beam C. The second angle θ' is selected to compensate for the decrease in the aspect ratio of interference pattern, resulting from the surface 18 arranged at the first angle θ.